The present invention relates to a polyester container obtained by biaxial-orientation blow-molding a preform made of polyester resin such as polyethylene terephthalate (PET). More particularly, the present invention relates to a polyester container in which food, such as baby food, and a drink, such as coffee with milk (cafxc3xa9 au lait), is filled, and which is sealed off and then treated by retort sterilization, as well as to a method of manufacturing the polyester container.
A polyester container in the form of a wide-mouthed jar or a bottle is manufactured by heating a preform made of polyester resin, such as polyethylene terephthalate, to a temperature not lower than the glass transition point (Tg), but not higher than the thermal crystallizing temperature, and then shaping the preform into the container by biaxial-orientation blow-molding. Because of being superior in transparency, shock resistance, gas barrier property, etc., such a polyester container is widely employed to contain a variety of foods, flavorings, drinks and so on.
However, unless a polyester container is crystallized during the biaxial-orientation blow-molding and is heat-set after the biaxial-orientation blow-molding at a temperature not lower than the crystallizing temperature to remove stress occurred in the molding step, the manufactured polyester container has insufficient heat stability and suffers remarkable deformation by heat shrinkage under a temperature condition of 70xc2x0 C. or higher.
To give a polyester container sufficient heat stability, therefore, a method has been employed in which a neck portion of a preform made of polyester resin is heated to an appropriate temperature for crystallization, and biaxial-orientation blow-molding is carried out on the preform in a mold held at a temperature around 140xc2x0 C. thereby heat-setting a molded piece. The polyester container manufactured by such a method, however, has a problem that, when retort sterilization is performed at, e.g., 120xc2x0 C. for 20-50 minutes on the container in which the contents have been filled, a bottom portion of the container shrinks and deforms. Further, in some cases, the deformation reaches a portion of the container body near the bottom, which may result in deformation of the container body and whitening of the bottom portion.
On the other hand, Japanese Patent Laid-open Publication No. 9-216,275 proposes a heat-resistant polyester container that is manufactured by the steps of whitening a neck portion of a preform; heating the preform to 100-120xc2x0 C. and forming a primary intermediate molded piece in a primary blowing mold held at a temperature higher than that of the preform; heating the obtained primary intermediate molded piece to 200-235xc2x0 C. to form a secondary intermediate molded piece as a result of heat shrinkage of the former; and blow-molding the secondary intermediate molded piece under biaxial orientation and, at the same time, heat-setting the body of the polyester container by utilization of the heat produced in the preceding step. In the polyester container manufactured by this prior art, however, the biaxial-orientation and the heat setting are not performed to a sufficient level in the bottom portion of the container. Accordingly, when the proposed container is subjected to retort sterilization at 120xc2x0 C. for 30 minutes, the bottom portion of the container is partly whitened.
The above-mentioned deformation in the bottom portion of the polyester container during the retort sterilization is attributable to stress remaining after the biaxial-orientation blow-molding in a secondary blowing mold, and is caused upon heat shrinkage of the container due to heating in the retort sterilization. On the other hand, the cause of whitening in the bottom portion of the polyester container resides in that, because a central area of the bottom portion is not sufficiently stretched during the step of biaxial-orientation blow-molding, polyester resin in the bottom central area is crystallized into spherical crystals due to heating in the retort sterilization.
For those reasons, the polyester containers proposed in the prior arts have a limitation in providing heat stability enough to withstand heat shrinkage caused at a filling temperature of approximately 95xc2x0 C. and in preventing the occurrence of whitening. In other words, it has been impossible to carry out the retort sterilization on those polyester containers at a high temperature not lower than 100xc2x0 C. particularly around 120xc2x0 C., for 20-50 minutes after filling foods, such as baby food, and drinks, such as coffee with milk, in the containers.
Accordingly, it is an object of the present invention to provide a highly heat-stable polyester container and a method of manufacturing the polyester container, which can be treated by retort sterilization at a high temperature after filling food, a drink or the like in the container and sealing off it, and which has superior heat stability enough to prevent deformation and whitening due to heat shrinkage in a bottom portion of the container even when treated by the retort sterilization.
To achieve the above object, according to the present invention, there is provided a polyester container in which a barrel portion and a bottom portion of the container are heat-set, and at least the bottom portion of the container has an endothermic peak on a DSC (Differential Scanning Calorimetry) curve in the range of not lower than about 150xc2x0 C. but not higher than a melting start point.
Also, according to the present invention, in the above polyester container, at least the bottom portion of the container has crystallinity not less than 35%.
Further, according to the present invention, there is provided a method of manufacturing a polyester container, which comprises the steps of blow-molding a preform made of polyester resin under biaxial orientation in a primary mold to obtain an intermediate molded piece having dimensions greater than a final molded product; heat-shrinking the intermediate molded piece; and blow-molding the heat-shrunk intermediate molded piece under biaxial orientation in a secondary mold and, at the same time, heat-setting a barrel portion and a bottom portion of the container in the secondary mold, thereby manufacturing a polyester container in which at least the bottom portion of the container has an endothermic peak on a DSC curve in the range of not lower than about 150xc2x0 C. but not higher than a melting start point.
Moreover, according to the present invention, in the above method of manufacturing the polyester container, at least the bottom portion of the container has crystallinity not less than 35%.